1. Field of the Invention
This invention relates generally to magnetic recording hard disk drives (HDDs), and more particularly to the HDD servo positioning system to locate and maintain the read/write heads on the data tracks.
2. Description of the Related Art
Magnetic recording hard disk drives (HDDs) use a servo-mechanical positioning system to hold the read/write head on the desired data track and to seek from track to track as required to perform read and write operations. Special “servo” information is written in fields in circumferentially-spaced servo sectors in each of the concentric data tracks on each disk surface. The servo sectors are constructed across multiple tracks to form angularly-spaced radially-directed servo sections that extend across the data tracks. As the servo sectors pass the read head, the readback signal is decoded to yield the position error signal (PES) that represents the radial position of the head. The servo sectors are typically written onto the disk during manufacturing in a process known as servowriting.
The servo sector includes a preamble, a servo address mark (SAM), a track identification (TID) field and a position error signal (PES) field. The preamble is typically an automatic gain control (AGC) field that is a regular series of transitions and is nominally the same at all radial positions. The AGC field allows the servo controller to calibrate gain parameters for the later TID and PES fields. The SAM field provides synchronization marks that allow the servo controller to synchronize with the later TID and PES fields. However, the servo writing process may result in track-to-track circumferential misalignment of the servo sectors. This does not generally present a problem during track following, i.e., when the read head is maintained on a single track because the frequency of the preamble transitions is constant. However, during a seek from one track to another track the read head has a radial velocity and may cross multiple tracks within a single servo section. If the misalignment of the preambles from track-to-track is too large, the servo system may not be able to acquire synchronization or may lose synchronization because the misalignment causes the frequency of the transitions in the SAM field to change with radial position.
What is needed is a HDD with a servo system that can adjust for frequency variations in the servo sector preamble transitions as the head crosses the servo sections during a track seek.